Heretofore, a printing ink has been produced by uniformly dispersing a printing ink composition composed of a pigment, which is obtained by subjecting giant needle stable type (β type) crystal particles having an average particle diameter of several to several hundred microns of a synthetic copper phthalocyanine to a microtization and pigmentation treatment thereby controlling the average particle diameter of the particles within a range from about 20 to 300 nm, in a varnish for a printing ink and a solvent for a printing ink using a kneading disperser such as a kneader, a roll mill or a beads mill. In this pigmentation treatment, a solvent salt milling method of adding a milling aid (common salt, etc.) and an organic solvent capable of promoting crystal transition into a stable (β type) crystal to a crude copper phthalocyanine and milling the mixture is widely used. However, this method requires an operation of separation and refinement of the pigmented copper phthalocyanine, the milling aid, and the organic solvent, and a large amount of waste water is generated, and thus an enormous amount of time and labors are required. As a method for solving this problem, there is proposed a method in which a dry-milled crude copper phthalocyanine is used as a pigment precursor which is a raw material for a printing ink (hereinafter referred to as a prepigment).
For example, Patent Document 1 describes a method for producing copper phthalocyanine suited for a pasty printing ink, which includes adding a resin to be contained in a completed printing ink in the proportion of 0.5 to 10% based on the amount of the pigment when a crude copper phthalocyanine is milled by a ball mill, and also describes the use of an alkyd resin, a hydrocarbon resin, a rosin resin modified with maleimide, or a rosin resin modified with phenol, as a resin.
Furthermore, Patent Document 2 describes a method for producing a printing ink containing copper phthalocyanine particles of a stable (β type) crystal having an aspect ratio of 1 to 2, which includes adding a resin for a printing ink to a crude copper phthalocyanine in the proportion of 20 to 80% by mass based on the copper phthalocyanine, dry-milling the mixture at 80 to 170° C., and heating the resulting milled product in a solvent or varnish for a printing ink.
These processes are methods which are advantageous in that the inhibitory effect of aggregation of copper phthalocyanine particles dry-milled by the resin added, and that the added resin containing the same components as those of the resin for a printing ink is used.
Furthermore, Patent Document 3 describes a method for producing a printing ink, which includes adding a mixture, which are obtained by adding a resin in the amount of 1 to 200% by mass based on a crude copper phthalocyanine and a solvent in the amount of 0.5 to 20% by mass based on the resin and dry-milling the mixture at 70 to 90° C., in a solvent or varnish for a printing ink, followed by a treatment, for the purpose of inhibiting conversion of pigment primary particles into a needle-shaped pigment and improving the reddish hue and fluidity of the ink.
Patent Document 4 describes a method of using a prepigment obtained by adding a resin, a solvent and a specific pigment derivative to a crude copper phthalocyanine and dry-milling the mixture at 60 to 180° C.
However, in the above methods using the resin, a large amount of resin must be added so as to obtain sufficient effects and there may arise adhesion of a milled product to the inside of a milling machine due to heat generation during dry-milling, and ignition due to heat storage, and also there arises a problem such as deterioration of hue, viscoelasticity and emulsifiability of a printing ink as a result of deterioration of performance of the resin due to heat or oxidation.
In order to solve these problems, Patent Document 5 describes a method for producing a printing ink, which includes using those obtained by dry-milling a crude copper phthalocyanine which is surface-treated with a rosin-based compound in advance and an extender pigment and/or a resin for a printing ink. It is described that the amount of the rosin-based compound used for the treatment is from 1 to 50% by mass based on the crude copper phthalocyanine, and the amount of the extender pigment and/or the resin for a printing ink is from 1 to 50% by mass based on the crude copper phthalocyanine
Patent Document 6 describes a method for producing a printing ink, which includes using a composition prepared from at least a crude copper phthalocyanine, an extender pigment in the amount of 1 to 100% by mass based on the crude copper phthalocyanine and a resin for a printing ink in the amount of 1 to 100% by mass based on the crude copper phthalocyanine.
However, in these methods, a resin for a printing ink and a rosin-based compound as a raw material of the resin for a printing ink are essential additive substances, and thus there still remain problems such as deterioration of productive efficiency due to adhesion to the inside of a dry milling machine and heat storage, ignition of the milled product, and quality deterioration of the printing ink due to quality deterioration of the resin.
The addition of plural substances such as a solvent, an extender pigment and a pigment derivative, together with the resin, leads to a limitation of the degree of freedom of design of a printing ink formula, and an increase in the costs of raw materials. Therefore, there has been a request to solve these problems.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Hei 2-294365    Patent Document 2: Japanese Patent No. 3,139,396    Patent Document 3: Japanese Patent No. 3,159,049    Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2006-206804    Patent Document 5: Japanese Patent No. 3,872,356    Patent Document 6: Japanese Unexamined Patent Application, First Publication No. 2003-41173